1. Field of Invention
This invention generally relates to process control and more specifically to a method for accurately and automatically maintaining the desired weight or thickness of a continuously extruded elastomer that may be applied as tread stock in the manufacture of vehicle tires.
2. Prior Art
Heretofore, the elastomeric stock used for tire treads is extruded in a continuous sheet form of the desired width, cooled, and cut to the required length for a particular tire circumference. The thickness of tread stock has heretofore been controlled by monitoring the weight of the extruded material since its weight is directly related to the material thickness.
Monitoring of the extrusion is accomplished by comparing the weight of a running length of continuous extrusion as it passes over a running-weigh-scale (hereinafter referred to as the RWS) to the weight of a cut length of the material taken at a check weight scale (hereinafter referred to as the CWS) after the extrusion is cooled and skived. The tread stock specification is established at the CWS with the RWS providing a means for making a first approximation inasmuch as the RWS and CWS are at separate locations in the process. Once the relationship between RWS and CWS is established, the desired value of CWS can be translated into a desired value of RWS. The desired value of RWS is the target value of RWS.
A method for the control of the thickness of extruded stock is disclosed in U.S. Pat. No. 4,088,721. In that patent, the RWS is continuously compared with the CWS to obtain a targeted average. The measured RWS is compared to the targeted average to obtain a control signal. The control signal is used to increase or decrease the speed of a take-away conveyor belt, positioned at the exit orifice of the extruder. If, for example, the measured RWS is too high compared to the targeted RWS, the speed of the take-away conveyor belt is increased to stretch the extrusion. This will result in a thinner tread stock. Alternatively, if the RWS value is too low, resulting in a low value of CWS, indicative of a thinner tread stock, the take-away belt may be slowed down with the result being a thicker extrusion.
While the method described offers simplicity of operation, it has a number of drawbacks. These will be apparent and the advantages of the present invention will be discussed in the detailed description that follows.